Hypogene sulfide precipitation during phyllic alteration: insights from copper isotopic evolution of the Dexing porphyry Cu–Mo–Au deposit, South China

The contributions of early potassic and later phyllic alteration stages to Cu endowment of the giant Dexing porphyry Cu–Mo–Au system in South China are determined using changes in the Cu isotope composition of hypogene chalcopyrite from three vein stages. The δ⁶⁵Cu values of chalcopyrite (δ⁶⁵Cu_cpy values) from the potassic (stage 1: -0.05‰ to 0.21‰) to the phyllic alteration stages (stage 2: -0.03‰ to 0.24‰) are relatively invariable, but chalcopyrite in the propylitic alteration stage (stage 3) has notably lower isotopic values (-0.35‰ to 0.02‰). The sharp decrease in δ⁶⁵Cu_cpy values in the latest vein stage is likely a result of precipitation of significant amounts of isotopically heavy chalcopyrite in the phyllic alteration environment. The overall isotopic evolution can be simulated, using a Rayleigh fractionation model, with the majority of Cu having precipitated during the phyllic alteration stage. By comparing our data with published Cu isotope results from other porphyry deposits, we demonstrate that the systematics of δ⁶⁵Cu_cpy values during different hydrothermal alteration stages could convincingly trace the relative timing and mechanism(s) of Cu precipitation in porphyry Cu systems. Spatial mapping of Cu isotopes at Dexing suggest that sharp decreases of δ⁶⁵Cu_cpy values in hypogene zones may be used to delineate the boundary of high-grade ore zones.